1. Field of the Invention
The invention relates to microprocessors in general and in particular, to their use as device controllers and interfaces for interconnecting the microprocessors.
2. Prior Art
The proliferation of personal computers (PCs), word processors, point of sale terminals (electronic cash registers), data terminal equipment (DTE) and the like have created a need for more efficient and faster Input/Output (I/O) devices. A conventional computer based terminal includes a control unit including a master microprocessor and different types of I/O devices coupled over appropriate communications lines or other medium to the control unit. Some of the well known I/O devices include keyboards, scanners, cathode my tubes (CRTs), etc.
The I/O devices form an interface between an operator (user) and the control unit. In this regard, the input device performs a physical act or undergoes a physical transformation to generate data which is packaged as messages and forwarded to the control unit for further processing. When the processing is completed, the control unit transmits the result to the output device which ultimately delivers the result to the operator or other user. For example, in case of a keyboard, the operator inputs data by activating selected keys. A control section of the keyboard monitors the keys and, as a character is depressed, an electronic representation of the depressed key is generated and forwarded to the main processor for further processing. Similarly, in the case of a scanner, information is scanned and electronic representations of the scanned information are generated and forwarded to the main processor for additional processing.
Broadly speaking, a typical input device has a structure to receive input information, a structure (usually electrical) to generate electrical representations of the input information and a communications interface including communications protocol for exchanging the information between the input device and the main processor of the control unit. In the case of a keyboard, the structure which receives input information includes the keys which the user activates to generate a desired document, etc. In the case of a scanner, the optical beam scans an input document or code and generates an electrical image which is subsequently forwarded to a master unit for further processing.
The prior art abounds with typical input devices. For example, U.S. Pat. No. 4,617,554 describes a multi-key electronic keyboard in which a keyboard processor scans the key to detect which ones are depressed, generates a character code representative of the depressed keys and forwards the code to a master CPU for further processing.
U.S. Pat. No. 4,706,068 discloses an interface structure for communicating between a keyboard and its terminal. The structure has a microprocessor in the terminal, four-wire interface at the keyboard and four wires interconnecting the keyboard and the terminal. The signals which are exchanged between the terminal and the keyboard include a keyboard initialize signal, a keyboard increment signal and a data signal. The initialize signal and increment signal are transmitted from the terminal to prime the keyboard to transmit data over the data line.
U.S. Pat. No. 4,766,418 discloses a keyboard wherein the keys of a "Qwerty" section are scanned by a first micro computer control circuit and the keys of a system select section are scanned by a second micro computer control circuit. The first microprocessor sends coded data representative of depressed "Qwerty" keys to the second microprocessor which, in turn, sends the coded data to the terminal. The second microprocessor also sends coded data representative of depressed system selection keys to the terminal. A data bus and an interrupt line and a control bus interconnect the two processors. In addition, a multiplicity of control lines interconnect the second micro computer to a "universal asynchronous receiver and transmitter" (UART) which interconnects the keyboard to the terminal.
Another prior art technique used for interprocessor communication is the well known "RS232" interface. The RS232 interface is a serial interface. However, the interface requires a plurality of hand shaking signals to be exchanged between the communicating devices prior to sending data on the communications line. A plurality of interconnecting control lines are required to transmit the hand shaking signals.
Even though the prior art devices and techniques work well for their intended purposes, each one is deficient in some aspect. The main areas of deficiencies are the controllers themselves and the communications method used to exchange information between microprocessors. Regarding the communications method, a relatively large number of ports or pins are required in order to provide control lines on which the hand shaking signals are transmitted. This creates unnecessary hardships where the controller is a microprocessor with a limited number of I/O pins. In addition, the hand shaking routine results in unnecessary overhead, with reduced system throughput. The controllers include cumbersome wire interfaces communicating with its terminal microprocessors or even if microprocessors are in the keyboards, their effectiveness is hampered by cumbersome communications protocols.